Pump stations are used especially in municipal engineering, where they are typically connected to pure water tanks, rain water tanks or waste water reservoirs. The pump station is then intended to prevent the tank/reservoir from being emptied or filled depending on the application. Pump stations often comprise a measurement apparatus for determining the liquid surface level by measuring the liquid surface level and controlling the pump on the basis of the surface level.
Pump stations used for liquid transfer are usually composed of one or more electrically driven pumps. The electric drive consists of a suitable current supply circuit, an electric motor and a control unit suitable for controlling and/or adjusting the electric motor. The pump operates as a load on the electric drive. The most frequently used electric motor in pump systems is an alternating-current motor, especially an induction motor. An alternating-current motor is most conveniently controlled by a contactor, and then the motor is switched on/off in accordance with the liquid surface level. However, the control unit often consists of a frequency converter because of the benefits yielded by this. The speed of an electric motor is controlled with a frequency converter, which converts the frequency of the voltage supplied to the motor. The frequency converter, in turn, is adjusted by appropriate electric control signals.
A prior art pump station is illustrated in FIG. 1. The pump 140 is electrically driven, the electric drive consisting of power supply 101, a frequency converter 120 acting as the control unit and an alternating-current motor 130, which is a three-phase motor in this case. The motor is usually connected to the pump with the rotation speed of the motor and the rotation speed of the pump being equal. The power supply 101 comprises an alternating-current network, such as a three-phase network, or any similar alternating-current source for supplying electric energy to the electric drive.
The pump station illustrated in FIG. 1 comprises a liquid tank 160, liquid 165 accumulated in this being pumped with a pump 140 into an exhaust manifold 142. The liquid surface level in the tank is measured by two surface level sensors 151 and 152, which are connected to the control unit 150. Each of the surface level sensors gives the control unit a signal indicating whether the surface level is above or below the sensor, in other words, the sensor is of switch type. The control unit 150 controls the pump operation e.g. as follows. When the liquid surface level is below the lower sensor 152, the pump is stopped. The pump remains switched off until the liquid surface reaches the upper surface level sensor 151, and then the pump is fully activated. The pump is operating until the liquid surface reaches the lower surface level sensor 152, and then the pump stops. There are also applications in which the pump is intended for pumping liquid into the tank and for keeping the liquid amount within given limits. In that case, the control described above has reverse operation, i.e. when the liquid surface level is below the lower surface level sensor, the pump is activated, and when the liquid surface level is above the upper surface level sensor, the pump stops. The functions described above do not utilise the feature of controlling the rotation speed provided by the frequency converter.
Instead of surface level sensors of switch type, one could use e.g. a surface level sensor 152 based on pressure measurement, the sensor being located at the bottom of the tank and providing information about the surface level at all surface levels. In that case, one often uses a control arrangement in which a constant surface level is aimed at, with the rotation speed of the pump being continually adjusted in accordance with the liquid amount entering the tank or consumed from the tank.
In FIG. 1, the pump and its drive have been illustrated outside the liquid tank for the sake of clarity, yet pump stations commonly use also pump installations within the liquid tank, e.g. at the tank bottom. Prior art arrangements have been described e.g. in patent specifications EP 619431 B1 and EP 100390 B1.
Prior art solutions involve a number of drawbacks. Separate installation of measurement and control apparatus requires work at the mounting site, and the appropriate mounting site and arrangement for the equipment and the sensors often require specific planning for each installation. The conditions at the mounting site may also vary, and this requires the use of measurement and control devices of different types depending on the conditions at the mounting site.
In addition, in prior art solutions, the energy consumption and efficiency of the pump station depends on external factors, e.g. on the flow-time distribution of the liquid entering a tank to be emptied or of the liquid consumed from a tank to be filled. Thus, a pump station may have poor energy consumption efficiency. In addition, the operating speed of the pump may be—especially in continuously regulated systems—permanently so low that impurities, which risk to cause obstructions, gather in the piping because of the low flow. The drawbacks mentioned above increase the cost of installing the pump station, of the equipment and of the operation.